POWER GENERATION AND TRANSMISSION 53
istics, the generators must have similar starting characteristics and their excitations must be adjusted to equalize power factors.
Alternating-current Motors.—Motor voltages are commonly 110, 220 or 440. There are three general types of motor: commutator, synchronous and induction. Commutator (single phase) motors are used for low voltages on railway and hoisting work. They cost 50 to 100 per cent more than direct-current motors. Their characteristics are like those of direct-current series motors: the efficiency is fairly high, but the power factor is low at low speeds. Synchronous polyphase motors give the highest efficiencies and power factors, are inexpensive and give steady speeds: but will not start under load: if single-phase, they will not start of themselves at all, and the efficiency is low.1 If heavily overloaded, they will stop. Direct current is used for field excitation. Voltages up to 13,000 may be used directly, in synchronous motors with revolving fields.
Induction motors are most commonly used, the stator being the primary. They are rated like generators, on the basis of temperature rise. They are essentially constant-speed machines. The squirrel-cage type requires a large starting current, and is started by fractional voltage taps. The wound-rotor type is preferred, generally, for applications where starts are frequent or severe. Sizes up to 200 hp. may have starting resistance mounted inside the rotor: large sizes have external resistance and the rotor is then provided with collector rings and brushes.2
Single-phase and two-phase induction motors usually have independent phase windings. Three-phase motors are connected in Y or A. In Y, the current per phase equals the line current and the voltage per phase is line voltage divided by \/3: in A, the voltage per phase equals line voltage and the current per phase is line current divided by \/3. In either case, current per phase is proportional to horsepower divided by (efficiency X voltage per phase X power factor). Polyphase induction motors have full-load efficiencies of 0.80 to 0.90 in sizes from 1 to 200 hp., 25-cycle. Corresponding power factors are 0.78 to 0.92. Sixty-cycle motors give efficiencies very slightly lower and full-load power factors about the same. The power factor decreases at light loads, especially for single-phase machines: to obtain high values implies high first cost of motor. Maximum efficiency and power factor are usually realized at about three-fourths rated load. The breakdown point (maximum safe load) should be at least 50 per cent in excess of the rating, and no load imposed should equal this.
Weights and costs vary with power and speed about as do those of direct-current motors. Large machines run at slow speeds. Voltage has little influence on cost. Starters cost two to three times as much as those of direct-current motors.
There are no generally applicable methods of speed variation for alternating-current motors. All methods involve a considerable sacrifice of efficiency.
Transformers.—Static transformers are ordinarily used to transform one alternating-current voltage to another, and are rated on the basis of temperature rise. The efficiency may be as high as 0.97 at full load. The curves show variation in efficiency and secondary voltage with load. The energy lost appears as heat and in large sizes is removed by surrounding the coils with oil, by pumping oil or water through or by an air blast. A separate transformer may be used on each phase of a polyphase system, or one three-phase unit may replace three single-phase transformers. The former arrangement may give more spare units: the latter decreases cost and increases efficiency. Costs increase slightly with primary voltage and vary inversely with frequency.
1 For starting, an auxiliary motor of another type may be used, or the synchronous motor may have a squirrel-cage winding in the pole face, so that it starts as an induction motor.
2 Single-phase motors may be started (a) by incorporating a commutator, (6) by hand and (c) by operating them as polyphase motors, a "phase splitter" being used to throw two or more circuits out of phase with each other. •